prefer const constructors (#8594)

* prefer const constructors

* fix double throw

examples/flutter_gallery/lib/demo/animation/home.dart

@@ -446,7 +446,7 @@ class _AnimationDemoHomeState extends State<AnimationDemoHome> {
     final List<Widget> headings = <Widget>[];
     for (int index = 0; index < allSections.length; index++) {
       headings.add(new Container(
-          decoration: new BoxDecoration(backgroundColor: _kAppBackgroundColor),
+          decoration: const BoxDecoration(backgroundColor: _kAppBackgroundColor),
           child: new ClipRect(
             child: new _AllSectionsView(
               sectionIndex: index,
@@ -526,8 +526,8 @@ class _AnimationDemoHomeState extends State<AnimationDemoHome> {
             top: statusBarHeight,
             left: 0.0,
             child: new IconTheme(
-              data: new IconThemeData(color: Colors.white),
-              child: new BackButton(),
+              data: const IconThemeData(color: Colors.white),
+              child: const BackButton(),
             ),
           ),
         ],

examples/flutter_gallery/lib/demo/cupertino/cupertino_dialog_demo.dart

@@ -57,7 +57,7 @@ class _CupertinoDialogDemoState extends State<CupertinoDialogDemo> {
                       onPressed: () { Navigator.pop(context, 'OK'); }
                     ),
                     new CupertinoDialogAction(
-                      child: new Text('Cancel', style: new TextStyle(fontWeight: FontWeight.w600)),
+                      child: new Text('Cancel', style: const TextStyle(fontWeight: FontWeight.w600)),
                       onPressed: () { Navigator.pop(context, 'Cancel'); }
                     ),
                   ]
@@ -69,7 +69,7 @@ class _CupertinoDialogDemoState extends State<CupertinoDialogDemo> {
           new CupertinoButton(
             child: new Text('Alert with Title'),
             color: CupertinoButton.kBlue,
-            padding: new EdgeInsets.symmetric(vertical: 16.0, horizontal: 36.0),
+            padding: const EdgeInsets.symmetric(vertical: 16.0, horizontal: 36.0),
             onPressed: () {
               showDemoDialog<String>(
                 context: context,

packages/flutter/lib/src/material/app_bar.dart

@@ -451,7 +451,7 @@ class _AppBarState extends State<AppBar> {
         children: <Widget>[
           new Flexible(
             child: new ConstrainedBox(
-              constraints: new BoxConstraints(maxHeight: kToolbarHeight),
+              constraints: const BoxConstraints(maxHeight: kToolbarHeight),
               child: appBar,
             ),
           ),

packages/flutter/lib/src/material/icon_button.dart

@@ -143,7 +143,7 @@ class IconButton extends StatelessWidget {
       currentColor = disabledColor ?? Theme.of(context).disabledColor;
 
     Widget result = new ConstrainedBox(
-      constraints: new BoxConstraints(minWidth: kMinButtonSize, minHeight: kMinButtonSize),
+      constraints: const BoxConstraints(minWidth: kMinButtonSize, minHeight: kMinButtonSize),
       child: new Padding(
         padding: padding,
         child: new SizedBox(

packages/flutter/lib/src/services/message_codecs.dart

@@ -205,7 +205,7 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
     final ReadBuffer buffer = new ReadBuffer(message);
     final dynamic result = _readValue(buffer);
     if (buffer.hasRemaining)
-      throw new FormatException('Message corrupted');
+      throw const FormatException('Message corrupted');
     return result;
   }
 
@@ -303,7 +303,7 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
 
   static dynamic _readValue(ReadBuffer buffer) {
     if (!buffer.hasRemaining)
-      throw throw new FormatException('Message corrupted');
+      throw const FormatException('Message corrupted');
     dynamic result;
     switch (buffer.getUint8()) {
       case _kNull:
@@ -363,7 +363,7 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
           result[_readValue(buffer)] = _readValue(buffer);
         }
         break;
-      default: throw new FormatException('Message corrupted');
+      default: throw const FormatException('Message corrupted');
     }
     return result;
   }
@@ -405,7 +405,7 @@ class StandardMethodCodec implements MethodCodec {
   dynamic decodeEnvelope(ByteData envelope) {
     // First byte is zero in success case, and non-zero otherwise.
     if (envelope == null || envelope.lengthInBytes == 0)
-      throw new FormatException('Expected envelope, got nothing');
+      throw const FormatException('Expected envelope, got nothing');
     final ReadBuffer buffer = new ReadBuffer(envelope);
     if (buffer.getUint8() == 0)
       return StandardMessageCodec._readValue(buffer);
@@ -415,6 +415,6 @@ class StandardMethodCodec implements MethodCodec {
     if (errorCode is String && (errorMessage == null || errorMessage is String))
       throw new PlatformException(code: errorCode, message: errorMessage, details: errorDetails);
     else
-      throw new FormatException('Invalid envelope');
+      throw const FormatException('Invalid envelope');
   }
 }

packages/flutter/test/cupertino/button_test.dart

@@ -47,7 +47,7 @@ void main() {
     await tester.pumpWidget(new Center(child: new CupertinoButton(
       child: new Text('X', style: testStyle),
       onPressed: null,
-      color: new Color(0xFFFFFFFF),
+      color: const Color(0xFFFFFFFF),
     )));
     final RenderBox buttonBox = tester.renderObject(find.byType(CupertinoButton));
     expect(
@@ -61,7 +61,7 @@ void main() {
     await tester.pumpWidget(new Center(child: new CupertinoButton(
       child: new Text(' ', style: testStyle),
       onPressed: null,
-      padding: new EdgeInsets.all(100.0),
+      padding: const EdgeInsets.all(100.0),
     )));
     final RenderBox buttonBox = tester.renderObject(find.byType(CupertinoButton));
     expect(

packages/flutter/test/material/app_bar_test.dart

@@ -311,8 +311,8 @@ void main() {
     );
 
     final Finder hamburger = find.byTooltip('Open navigation menu');
-    expect(tester.getTopLeft(hamburger), new Point(0.0, 0.0));
-    expect(tester.getSize(hamburger), new Size(56.0, 56.0));
+    expect(tester.getTopLeft(hamburger), const Point(0.0, 0.0));
+    expect(tester.getSize(hamburger), const Size(56.0, 56.0));
   });
 
   testWidgets('test action is 4dp from edge and 48dp min', (WidgetTester tester) async {
@@ -343,13 +343,13 @@ void main() {
 
     final Finder addButton = find.byTooltip('Add');
     // Right padding is 4dp.
-    expect(tester.getTopRight(addButton), new Point(800.0 - 4.0, 0.0));
+    expect(tester.getTopRight(addButton), const Point(800.0 - 4.0, 0.0));
     // It's still the size it was plus the 2 * 8dp padding from IconButton.
-    expect(tester.getSize(addButton), new Size(60.0 + 2 * 8.0, 56.0));
+    expect(tester.getSize(addButton), const Size(60.0 + 2 * 8.0, 56.0));
 
     final Finder shareButton = find.byTooltip('Share');
     // The 20dp icon is expanded to fill the IconButton's touch target to 48dp.
-    expect(tester.getSize(shareButton), new Size(48.0, 56.0));
+    expect(tester.getSize(shareButton), const Size(48.0, 56.0));
   });
 
   testWidgets('SliverAppBar default configuration', (WidgetTester tester) async {

packages/flutter/test/material/icon_button_test.dart

@@ -33,7 +33,7 @@ void main() {
     );
 
     final RenderBox iconButton = tester.renderObject(find.byType(IconButton));
-    expect(iconButton.size, new Size(48.0, 48.0));
+    expect(iconButton.size, const Size(48.0, 48.0));
 
     await tester.tap(find.byType(IconButton));
     expect(mockOnPressedFunction.called, 1);
@@ -53,7 +53,7 @@ void main() {
     );
 
     final RenderBox iconButton = tester.renderObject(find.byType(IconButton));
-    expect(iconButton.size, new Size(48.0, 48.0));
+    expect(iconButton.size, const Size(48.0, 48.0));
   });
 
   testWidgets('test icons can be small when total size is >48dp', (WidgetTester tester) async {
@@ -62,7 +62,7 @@ void main() {
         child: new Center(
           child: new IconButton(
             iconSize: 10.0,
-            padding: new EdgeInsets.all(30.0),
+            padding: const EdgeInsets.all(30.0),
             onPressed: mockOnPressedFunction,
             icon: new Icon(Icons.link),
           ),
@@ -71,7 +71,7 @@ void main() {
     );
 
     final RenderBox iconButton = tester.renderObject(find.byType(IconButton));
-    expect(iconButton.size, new Size(70.0, 70.0));
+    expect(iconButton.size, const Size(70.0, 70.0));
   });
 
   testWidgets('test default icon buttons are constrained', (WidgetTester tester) async {
@@ -89,7 +89,7 @@ void main() {
     );
 
     final RenderBox box = tester.renderObject(find.byType(IconButton));
-    expect(box.size, new Size(80.0, 80.0));
+    expect(box.size, const Size(80.0, 80.0));
   });
 
   testWidgets(
@@ -110,7 +110,7 @@ void main() {
     );
 
     final RenderBox box = tester.renderObject(find.byType(IconButton));
-    expect(box.size, new Size(48.0, 600.0));
+    expect(box.size, const Size(48.0, 600.0));
   });
 
   testWidgets('test default padding', (WidgetTester tester) async {
@@ -127,7 +127,7 @@ void main() {
     );
 
     final RenderBox box = tester.renderObject(find.byType(IconButton));
-    expect(box.size, new Size(96.0, 96.0));
+    expect(box.size, const Size(96.0, 96.0));
   });
 
   testWidgets('test tooltip', (WidgetTester tester) async {

packages/flutter/test/material/scaffold_test.dart

@@ -44,7 +44,7 @@ void main() {
   testWidgets('Scaffold large bottom padding test', (WidgetTester tester) async {
     final Key bodyKey = new UniqueKey();
     await tester.pumpWidget(new MediaQuery(
-      data: new MediaQueryData(
+      data: const MediaQueryData(
         padding: const EdgeInsets.only(bottom: 700.0),
       ),
       child: new Scaffold(
@@ -56,7 +56,7 @@ void main() {
     expect(bodyBox.size, equals(const Size(800.0, 0.0)));
 
     await tester.pumpWidget(new MediaQuery(
-      data: new MediaQueryData(
+      data: const MediaQueryData(
         padding: const EdgeInsets.only(bottom: 500.0),
       ),
       child: new Scaffold(
@@ -67,7 +67,7 @@ void main() {
     expect(bodyBox.size, equals(const Size(800.0, 100.0)));
 
     await tester.pumpWidget(new MediaQuery(
-      data: new MediaQueryData(
+      data: const MediaQueryData(
         padding: const EdgeInsets.only(bottom: 580.0),
       ),
       child: new Scaffold(

packages/flutter/test/widgets/scroll_view_test.dart

@@ -269,7 +269,7 @@ void main() {
         primary: true,
         children: <Widget>[
           new Container(
-            constraints: new BoxConstraints(maxHeight: 200.0),
+            constraints: const BoxConstraints(maxHeight: 200.0),
             child: new ListView(key: innerKey, primary: true),
           ),
         ],

packages/flutter/test/widgets/single_child_scroll_view_test.dart

@@ -163,7 +163,7 @@ void main() {
       child: new SingleChildScrollView(
         primary: true,
         child: new Container(
-          constraints: new BoxConstraints(maxHeight: 200.0),
+          constraints: const BoxConstraints(maxHeight: 200.0),
           child: new ListView(key: innerKey, primary: true),
         ),
       ),

packages/flutter_tools/lib/executable.dart

@@ -238,7 +238,7 @@ Future<int> _handleToolError(
 /// be recording. Additionally, in the case of a crash we do not trust the
 /// integrity of the [AppContext].
 @visibleForTesting
-FileSystem crashFileSystem = new LocalFileSystem();
+FileSystem crashFileSystem = const LocalFileSystem();
 
 /// Saves the crash report to a local file.
 Future<File> _createLocalCrashReport(List<String> args, dynamic error, Chain chain) async {

packages/flutter_tools/lib/src/vmservice_record_replay.dart

@@ -36,7 +36,7 @@ class RecordingVMServiceChannel extends DelegatingStreamChannel<String> {
       _messages.sort();
 
       final File file = _getManifest(location);
-      final String json = new JsonEncoder.withIndent('  ').convert(_messages);
+      final String json = const JsonEncoder.withIndent('  ').convert(_messages);
       await file.writeAsString(json, flush: true);
     }, ShutdownStage.SERIALIZE_RECORDING);
   }

packages/flutter_tools/test/crash_reporting_test.dart

@@ -28,7 +28,7 @@ void main() {
     });
 
     tearDown(() {
-      tools.crashFileSystem = new LocalFileSystem();
+      tools.crashFileSystem = const LocalFileSystem();
       restoreExitFunction();
       exitTestingMode();
     });